As Whappo has asked, more information on your setup and what you have tried to access the drives would be most helpful. Is there any data you need on the drives, or can you re-format the drives if necessary?
Here are some suggestions if you are running WinXP as Administrator to try in the meantime:
Make sure "Show hidden files" is enabled in Folder Options.
Open Windows Disk Management and change the drive letter for the usb drive, also make sure the Capacity shown in Disk Management matches the capacity of your usb drive,
then try to access with:
1. right-click the drive in My Computer and "Explore", If successful look for and delete any autorun.inf file.
Use the Run window with: K:/ (note--change K to the to the drive letter assigned to the usb drive.) then the same as 1. above, delete any autorun.inf.
Use the Run window with: chkdsk /k (note--change k to the to the drive letter assigned to the usb drive.)
In Disk Management, select the usb drive and try to re-format it.
All this assumes you have a clean PC(no virus or malware).